For common tapping screws, appropriate tightening may fail to be performed on steel sheets with a sheet thickness of less than 1 mm. The reason for the failure will be explained with reference to FIGS. 11 to 15. FIG. 11 is a front view of a tapping screw according to a conventional example. FIG. 12 is a CC cross-sectional view in FIG. 11. FIGS. 13 to 15 are diagrams depicting how the tapping screw according to the conventional example is tightened.
A tapping screw 500 includes a threaded shank 510 and a head 520. In general, single-start thread is formed on the shank 510 of the tapping screw 500. As depicted in the cross-sectional view in FIG. 12, the cross-sectional shape of the tapping screw 500 is a noncircular cross section resulting from composition of a circle of the ridge and a circle of a root. In FIG. 12, a solid line V corresponds to a root diameter, and a solid line S corresponds to an external shape of the external thread. When the thus configured tapping screw 500 is used to, for example, fix two sheet-like members (fix a tightening target member to a screwed member), a prepared hole is preformed in the screwed member, and an insertion hole is preformed in the tightening target member. Then, the tapping screw is driven into the prepared hole in the screwed member via the insertion hole formed in the tightening target member side to allow the two members to be fixed. In this case, when the thickness of the screwed member is sufficient, even if the cross section of the shank 510 is a noncircular cross section, an area of the ridge which is equal to or larger than one circumference advances in a continuous spiral manner while in contact with the prepared hole to form an internal thread. Thus, the shank 510 is driven while remaining perpendicular to the two members.
In this regard, a case will be described where the tapping screw 500 configured as described above is assumed to be used when a second steel sheet 300 is fixed to a first steel sheet 200 with a small sheet thickness as depicted in FIGS. 13 to 15. A prepared hole 210 is preformed in the first steel sheet 200. An insertion hole 310 through which the shank 510 of the tapping screw 500 is inserted is preformed in the second steel sheet 300. Since the first steel sheet 200 is thin (for example, approximately 0.4 mm), the area in which the ridge formed on the shank 510 of the tapping screw 500 contacts the prepared hole 210 is smaller than one circumference. Consequently, the length of the spiral internal thread formed is, for example, approximately half a circumference. Thus, as described above, since the cross section of the shank 510 is a noncircular cross section, the shank 510 is obliquely tilted while being driven into the first steel sheet 200 and the second steel sheet 300. Furthermore, when a bearing surface 521 of the head 520 comes into abutting contact with the second steel sheet 300, the ridge also contacts the prepared hole 210 along approximately half a circumference. Not only is the tapping screw 500 tilted but also a sufficient tightening force is not obtained.
Furthermore, in general, a threaded portion near and below a neck of an external threaded fastener is referred to as an incomplete thread, which has an incomplete thread shape. When two steel sheets each with a small sheet thickness are tightened together as described above, the incomplete thread may be fitted into an internal threaded portion formed. In this case, the strength of a portion of fitting between the fitting portion between the external threaded portion and the internal threaded portion with respect to an axial load decreases significantly. In connection with this, workability at the time of tightening may be degraded, and the internal threaded portion may be destroyed even with a relatively weak external force. Additionally, the tapping screw may be loosened.
As described above, conventionally, when the screwed member and the tightening target member are thin like steel sheets, directly using the tapping screw 500 is impossible. Thus, for example, fixation is performed using a bolt and a nut, or steel sheets are burred to increase the number of threads formed by the external thread before the tapping screw is used.
A tightening method using a bolt and a nut increases the number of components, and a method of burring steel sheets increases the number of processing operations and needs a complicated mold. Thus, costs increase in both cases. Furthermore, conventionally, burring of steel sheets disadvantageously involves a failure to achieve a sufficient height of burring. Consequently, there has been a desire for a tapping screw that can be used for a thin screwed member and a thin tightening target member without the need, for example, to burr the members.    Patent Document 1: Japanese Patent Application Laid-open No. 2009-150534    Patent Document 2: Japanese Patent Application Laid-open No. H10-131932    Patent Document 3: Japanese Patent No. 4260624    Patent Document 4: Japanese Patent No. 4480395